The present invention relates to a transducer for converting a physical quantity such as an electric current, an electric voltage, or a temperature or a pressure or the like into a linearized output which is restricted within a certain range, and more particularly relates to such a transducer which incorporates into itself some of the functions of a meter relay.
A conventional type of transducer, taking the exemplary case of for instance a voltage transducer, converts an inputted voltage of a potential transformer (referred to as PT hereinafter), which may range over the AC voltage range of for example about 0 to about 150 volts AC, into a DC voltage range of for example about 0 to about 5 volts DC, while on the other hand a current transducer converts an electric current obtained from a current transformer (referred to as CT hereinafter) into a DC voltage which may similarly vary over a certain range. Thus, the output of the transducer corresponds to the inputted physical quantity; and, when additionally it is required not only to display the input signal value but also an upper and a lower limit value for said input signal are required to be set up, and when it is further required to produce an output signal when the input signal value passes either of said limit values, i.e. exceeds the higher such limit value or drops below the lower such limit value, a so called meter relay is additionally required. However, since such a meter relay and such a transducer are, conventionally, different devices, and since they are required to be wired into a circuit individually, the problems have arisen that the wiring tends to become complicated, and the space required for installation of the combination tends to become rather substantial. Furthermore, since some disagreement may occur between the output voltage of the transducer and the upper and the lower limit values at which the meter relay is activated, the level of actual practical usability may not be as high as desirable. Additionally, when a control system is formed by applying such a transducer and such a meter relay to a sequencer or the like, the adjustment of the overall system is difficult unless an output signal of substantially predetermined specifications is supplied by the transducer to the system.
Further, in such a transducer, the problem has arisen in the prior art that the difference between the current value and the lower limit, or the difference between the current value and the upper limit, cannot be readily recognized or displayed.
Another desirable feature for such a transducer would be to have as high a current handling capacity as possible, or alternatively to be able to control two independent circuits at one time.
A yet further desirable feature for such a transducer would be for the setting of the values for the upper and the lower limit to be ensured to be foolproof. In other words, it would be desirable for it not to be possible to input a mistaken value for the upper limit or for the lower limit, and for transducer operation to continue based upon such a mistaken value. In the prior art, the inputting of such and upper limit or of such a lower limit has been conventionally performed by the use of a thumb wheel type rotary switch or the like, and no practical form of restriction upon such a rotary switch can usefully be imposed.
Further, this type of transducer incorporates, typically, one or more A/D converter circuits. Now, such an A/D converter circuit may be a so called AC effective value-DC conversion circuit, which is a circuit for finding the effective value of an AC signal and for converting it into a DC signal according to the effective value. A conventional AC effective value-DC conversion circuit comprises a squaring circuit for squaring an input signal, an averaging circuit for averaging the output of the squaring circuit by integrating it, and a square rooting circuit for square rooting the output of the averaging circuit, and is known in, for example, a form which is incorporated into a monolithic IC. Such a structure is detailed in, for example, pages 421 to 422 of the publication "Jitsuyo Denshi Handbook (4), Fifth Edition, which was published on approximately Nov. 1, 1983 by CQ Shuppan KK.
However, since such a conventional AC effective value-DC conversion circuit which is incorporated into a monolithic IC has required a square rooting circuit, its circuit structure has tended to be extremely complex and was therefore expensive. Furthermore, in spite of such high cost, its reliability has not proved to be quite satisfactory.
A previously proposed AC effective value-DC conversion circuit of another type comprises a rectifier and a time constant circuit consisting of a resistor and a capacitor, and conducts a pseudo effective value conversion by setting the time constant of the time constant circuit to a certain special value. This conversion circuit has the advantage of simplicity but is not precise in its conversion accuracy, making it unsuitable for precise effective value conversion.
Also, a requirement has arisen for an improved ripple removal circuit which can be applied when an AC signal is converted into a DC signal and the DC signal is supplied to an AD converter so that a certain appliance can be digitally controlled for instance in an AC effective value-DC conversion circuit. In the prior art, in the case of such an original signal, if an AC component is superimposed on the DC component of the original signal and the level of the ripple thereof is substantial, conversion errors in AD connversion can happen by confusing "L" level for "H" level. Therefore, conventionally, to the end of removing such a ripple, the rectified output from a rectifying circuit for converting an AC signal into a DC signal has been typically smoothed by a smoothing capacitor. However, in such a conventional situation, the capacitance of the capacitor is desired to be high to the end of better removing the ripple, but, as shown in FIG. 26 of the accompanying drawings, if the capacitance of the capacitor is excessive and the amplitude of the AC component which is inputted to the rectifying circuit is varied, the response to the amplitude variation is impaired and control errors may occur when highly timewise precise control is attempted, due to errors in the timing of the switch over between "L" and "H" level. Such errors can be reduced by reducing the capacitance of the capacitor but in that case the ripple can not be effectively removed. Thus, the removal of ripples and a high control response present themselves as two conflicting objectives when the ripples are to be removed by a smoothing capacitor, and it has in the prior art been impossible to accomplish the two objectives at the same time.